In recent years, there has been progress in portable electronic devices that are compact in design, such as cellular phones and hearing aids. Among such portable electronic devices, there is a gradual increase in demand for devices that operate in contact with a living body. For example, development is in progress for a biological information signal generating device capable of measuring and monitoring biological information such as body temperature, blood pressure, and pulse, and automatically notifying such biological information to facilities such as hospitals. Moreover, development is also in progress for a body-pasting device capable of supplying medicine, etc. through the outer skin of a living body by application of voltage.
Under such circumstances, a compact design is also demanded of batteries which supply power to the portable electronic devices. That is, thin and flexible batteries are in demand. Examples of thin batteries that have been developed until now, include paper batteries, thin flat batteries, and plate batteries. However, since such batteries use high-strength housings, there is the problem of having difficulty in making the batteries more flexible and thin.
Therefore, development is in progress for a thin battery which uses a thin, flexible laminate film for the housing (c.f., Patent Literatures 1 and 2). For example, to obtain a thin battery, a positive electrode and a negative electrode, both in sheet form, are stacked with a separator interposed therebetween; a positive lead and a negative lead are connected to the positive electrode and the negative electrode, respectively; and the resultant is wrapped with a laminate film, such that the leads are partially exposed to the outside.
A thin battery which uses a laminate film for its housing can be formed thinner than a conventional thin battery, and is therefore advantageous in terms of energy density. Moreover, the stacked structure of the electrodes connected in series or in parallel inside the housing, facilitates improvements in voltage and capacity.
Here, in the electrode assembly for a thin battery, the positive electrode and the negative electrode are integrated together by having an electrolyte layer interposed therebetween, thereby allowing the positive and negative electrodes to adhere to each other. Adhesion between the electrodes is important for realizing improvements in energy density and high-load characteristics, and reduction in internal resistance. That is, reduced adhesion between the electrodes would cause significant deterioration in various characteristics of the battery.
Therefore, in the process for producing a thin battery, when performing thermal welding to seal the housing accommodating the electrode assembly, the housing and the electrode assembly are machine pressed under reduced pressure. This is considered to suppress creation of space between the housing and the electrode assembly, and to thus facilitate maintaining adhesion between the electrodes (c.f., Patent Literatures 3 and 4).